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        redis sds 对齐理解
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    <p>Redis sds 的源码中，使用了 <code>__attribute__ ((__packed__))</code> ，一般情况下，结构体会按其所有变量大小的最小公倍数做字节对齐，而用<code>packed</code>修饰以后，结构体则变为按1字节进行对齐。这个最小公倍数如何进行理解呢？</p>

<p>以以下结构进行举例：</p>
<div class="highlight"><pre style="background-color:#f8f8f8;-moz-tab-size:4;-o-tab-size:4;tab-size:4"><code class="language-c" data-lang="c"><span style="color:#8f5902;font-style:italic">#</span><span style="color:#8f5902;font-style:italic">include</span> <span style="color:#8f5902;font-style:italic">&lt;stdio.h&gt;</span><span style="color:#8f5902;font-style:italic">
</span><span style="color:#8f5902;font-style:italic"></span>
<span style="color:#204a87;font-weight:bold">typedef</span> <span style="color:#204a87;font-weight:bold">struct</span> <span style="color:#000">MemAlign</span>  
<span style="color:#000;font-weight:bold">{</span>  
    <span style="color:#204a87;font-weight:bold">char</span> <span style="color:#000">a</span><span style="color:#000;font-weight:bold">[</span><span style="color:#0000cf;font-weight:bold">18</span><span style="color:#000;font-weight:bold">]</span><span style="color:#000;font-weight:bold">;</span>  
    <span style="color:#204a87;font-weight:bold">double</span> <span style="color:#000">b</span><span style="color:#000;font-weight:bold">;</span>     
    <span style="color:#204a87;font-weight:bold">char</span> <span style="color:#000">c</span><span style="color:#000;font-weight:bold">;</span>  
    <span style="color:#204a87;font-weight:bold">int</span> <span style="color:#000">d</span><span style="color:#000;font-weight:bold">;</span>    
    <span style="color:#204a87;font-weight:bold">short</span> <span style="color:#000">e</span><span style="color:#000;font-weight:bold">;</span>      
<span style="color:#000;font-weight:bold">}</span><span style="color:#000">MemAlign</span><span style="color:#000;font-weight:bold">;</span>

<span style="color:#204a87;font-weight:bold">typedef</span> <span style="color:#204a87;font-weight:bold">struct</span> <span style="color:#000">__attribute__</span> <span style="color:#000;font-weight:bold">(</span><span style="color:#000;font-weight:bold">(</span><span style="color:#000">__packed__</span><span style="color:#000;font-weight:bold">)</span><span style="color:#000;font-weight:bold">)</span>  <span style="color:#000">MemAlignPacked</span>  
<span style="color:#000;font-weight:bold">{</span>  
    <span style="color:#204a87;font-weight:bold">char</span> <span style="color:#000">a</span><span style="color:#000;font-weight:bold">[</span><span style="color:#0000cf;font-weight:bold">18</span><span style="color:#000;font-weight:bold">]</span><span style="color:#000;font-weight:bold">;</span>  
    <span style="color:#204a87;font-weight:bold">double</span> <span style="color:#000">b</span><span style="color:#000;font-weight:bold">;</span>     
    <span style="color:#204a87;font-weight:bold">char</span> <span style="color:#000">c</span><span style="color:#000;font-weight:bold">;</span>  
    <span style="color:#204a87;font-weight:bold">int</span> <span style="color:#000">d</span><span style="color:#000;font-weight:bold">;</span>    
    <span style="color:#204a87;font-weight:bold">short</span> <span style="color:#000">e</span><span style="color:#000;font-weight:bold">;</span>      
<span style="color:#000;font-weight:bold">}</span><span style="color:#000">MemAlignPacked</span><span style="color:#000;font-weight:bold">;</span>  

<span style="color:#204a87;font-weight:bold">int</span> <span style="color:#000">main</span><span style="color:#000;font-weight:bold">(</span><span style="color:#204a87;font-weight:bold">void</span><span style="color:#000;font-weight:bold">)</span><span style="color:#000;font-weight:bold">{</span>
    <span style="color:#204a87;font-weight:bold">struct</span> <span style="color:#000">MemAlign</span> <span style="color:#000">m</span><span style="color:#000;font-weight:bold">;</span>
    <span style="color:#204a87;font-weight:bold">struct</span> <span style="color:#000">MemAlignPacked</span> <span style="color:#000">mp</span><span style="color:#000;font-weight:bold">;</span>
    <span style="color:#000">printf</span><span style="color:#000;font-weight:bold">(</span><span style="color:#4e9a06"></span><span style="color:#4e9a06">&#34;</span><span style="color:#4e9a06">========&gt;%d</span><span style="color:#4e9a06">&#34;</span><span style="color:#000;font-weight:bold">,</span> <span style="color:#204a87;font-weight:bold">sizeof</span><span style="color:#000;font-weight:bold">(</span><span style="color:#000">m</span><span style="color:#000;font-weight:bold">)</span><span style="color:#000;font-weight:bold">)</span><span style="color:#000;font-weight:bold">;</span>
    <span style="color:#000">printf</span><span style="color:#000;font-weight:bold">(</span><span style="color:#4e9a06"></span><span style="color:#4e9a06">&#34;</span><span style="color:#4e9a06">========&gt;%d</span><span style="color:#4e9a06">&#34;</span><span style="color:#000;font-weight:bold">,</span> <span style="color:#204a87;font-weight:bold">sizeof</span><span style="color:#000;font-weight:bold">(</span><span style="color:#000">mp</span><span style="color:#000;font-weight:bold">)</span><span style="color:#000;font-weight:bold">)</span><span style="color:#000;font-weight:bold">;</span>
<span style="color:#000;font-weight:bold">}</span></code></pre></div>
<p>内存存放数据是为了给CPU进行使用，而CPU访问内存数据时会受到地址总线宽度的限制，并且CPU从内存中获取数据时起始地址必须是地址总线宽度的倍数，也就是说CPU是将内存分为一块一块的，块的大小取决于地址总线宽度，并且CPU读取数据也是一块一块读取的，块的大小称为（memory granularity）内存读取粒度。</p>

<p>上面的结构中，有<code>char a[18]</code>这个属性，说明这个属性是占个字节的，在32位CPU的内存模型中，以4个字节对齐（如下图1），属性<code>char a[18]</code>因为占了18个字节，会需要补齐2个字节达到4的最小公倍数20，然后接着分配<code>double b</code>的内存地址（如下图2）。而使用了<code>__packed__</code>修饰以后，就不需要补齐。</p>

<div  align="center"><img src="https://wejack639.oss-cn-beijing.aliyuncs.com/blogimages/img/20221104105340.png" width = 400 /></div>

<div  align="center"><img src="https://wejack639.oss-cn-beijing.aliyuncs.com/blogimages/img/20221104183822.png" width = 500 /> </div>

<p>而在64位CPU的内存模型中，以8个字节进行对齐（如下图1）。则案例结构如下图2。我的mac是64位的，跑文章开始的代码跑出的结果和猜测的一样。</p>

<div  align="center"><img src="https://wejack639.oss-cn-beijing.aliyuncs.com/blogimages/img/20221104123453.png" width = 500 /> </div>

<div  align="center"><img src="https://wejack639.oss-cn-beijing.aliyuncs.com/blogimages/img/20221104183943.png" width = 500 /> </div>

<p>所以在redis的sds的源码中，出现的sdshdr5 、sdshdr8、sdshdr16、sdshdr32、sdshdr64，都使用了  <code>__attribute__ ((__packed__))</code>进行紧凑排列。好处在于节省内存和方便buf[-1]取到flags地址，获取类型更加方便。</p>
<div class="highlight"><pre style="background-color:#f8f8f8;-moz-tab-size:4;-o-tab-size:4;tab-size:4"><code class="language-c" data-lang="c"><span style="color:#8f5902;font-style:italic">/* Note: sdshdr5 is never used, we just access the flags byte directly.
</span><span style="color:#8f5902;font-style:italic"> * However is here to document the layout of type 5 SDS strings. */</span>
<span style="color:#204a87;font-weight:bold">struct</span> <span style="color:#000">__attribute__</span> <span style="color:#000;font-weight:bold">(</span><span style="color:#000;font-weight:bold">(</span><span style="color:#000">__packed__</span><span style="color:#000;font-weight:bold">)</span><span style="color:#000;font-weight:bold">)</span> <span style="color:#000">sdshdr5</span> <span style="color:#000;font-weight:bold">{</span>
    <span style="color:#204a87;font-weight:bold">unsigned</span> <span style="color:#204a87;font-weight:bold">char</span> <span style="color:#000">flags</span><span style="color:#000;font-weight:bold">;</span> <span style="color:#8f5902;font-style:italic">/* 3 lsb of type, and 5 msb of string length */</span>
    <span style="color:#204a87;font-weight:bold">char</span> <span style="color:#000">buf</span><span style="color:#000;font-weight:bold">[</span><span style="color:#000;font-weight:bold">]</span><span style="color:#000;font-weight:bold">;</span>
<span style="color:#000;font-weight:bold">}</span><span style="color:#000;font-weight:bold">;</span>
<span style="color:#204a87;font-weight:bold">struct</span> <span style="color:#000">__attribute__</span> <span style="color:#000;font-weight:bold">(</span><span style="color:#000;font-weight:bold">(</span><span style="color:#000">__packed__</span><span style="color:#000;font-weight:bold">)</span><span style="color:#000;font-weight:bold">)</span> <span style="color:#000">sdshdr8</span> <span style="color:#000;font-weight:bold">{</span>
    <span style="color:#000">uint8_t</span> <span style="color:#000">len</span><span style="color:#000;font-weight:bold">;</span> <span style="color:#8f5902;font-style:italic">/* used */</span>
    <span style="color:#000">uint8_t</span> <span style="color:#000">alloc</span><span style="color:#000;font-weight:bold">;</span> <span style="color:#8f5902;font-style:italic">/* excluding the header and null terminator */</span>
    <span style="color:#204a87;font-weight:bold">unsigned</span> <span style="color:#204a87;font-weight:bold">char</span> <span style="color:#000">flags</span><span style="color:#000;font-weight:bold">;</span> <span style="color:#8f5902;font-style:italic">/* 3 lsb of type, 5 unused bits */</span>
    <span style="color:#204a87;font-weight:bold">char</span> <span style="color:#000">buf</span><span style="color:#000;font-weight:bold">[</span><span style="color:#000;font-weight:bold">]</span><span style="color:#000;font-weight:bold">;</span>
<span style="color:#000;font-weight:bold">}</span><span style="color:#000;font-weight:bold">;</span>
<span style="color:#204a87;font-weight:bold">struct</span> <span style="color:#000">__attribute__</span> <span style="color:#000;font-weight:bold">(</span><span style="color:#000;font-weight:bold">(</span><span style="color:#000">__packed__</span><span style="color:#000;font-weight:bold">)</span><span style="color:#000;font-weight:bold">)</span> <span style="color:#000">sdshdr16</span> <span style="color:#000;font-weight:bold">{</span>
    <span style="color:#000">uint16_t</span> <span style="color:#000">len</span><span style="color:#000;font-weight:bold">;</span> <span style="color:#8f5902;font-style:italic">/* used */</span>
    <span style="color:#000">uint16_t</span> <span style="color:#000">alloc</span><span style="color:#000;font-weight:bold">;</span> <span style="color:#8f5902;font-style:italic">/* excluding the header and null terminator */</span>
    <span style="color:#204a87;font-weight:bold">unsigned</span> <span style="color:#204a87;font-weight:bold">char</span> <span style="color:#000">flags</span><span style="color:#000;font-weight:bold">;</span> <span style="color:#8f5902;font-style:italic">/* 3 lsb of type, 5 unused bits */</span>
    <span style="color:#204a87;font-weight:bold">char</span> <span style="color:#000">buf</span><span style="color:#000;font-weight:bold">[</span><span style="color:#000;font-weight:bold">]</span><span style="color:#000;font-weight:bold">;</span>
<span style="color:#000;font-weight:bold">}</span><span style="color:#000;font-weight:bold">;</span>
<span style="color:#204a87;font-weight:bold">struct</span> <span style="color:#000">__attribute__</span> <span style="color:#000;font-weight:bold">(</span><span style="color:#000;font-weight:bold">(</span><span style="color:#000">__packed__</span><span style="color:#000;font-weight:bold">)</span><span style="color:#000;font-weight:bold">)</span> <span style="color:#000">sdshdr32</span> <span style="color:#000;font-weight:bold">{</span>
    <span style="color:#000">uint32_t</span> <span style="color:#000">len</span><span style="color:#000;font-weight:bold">;</span> <span style="color:#8f5902;font-style:italic">/* used */</span>
    <span style="color:#000">uint32_t</span> <span style="color:#000">alloc</span><span style="color:#000;font-weight:bold">;</span> <span style="color:#8f5902;font-style:italic">/* excluding the header and null terminator */</span>
    <span style="color:#204a87;font-weight:bold">unsigned</span> <span style="color:#204a87;font-weight:bold">char</span> <span style="color:#000">flags</span><span style="color:#000;font-weight:bold">;</span> <span style="color:#8f5902;font-style:italic">/* 3 lsb of type, 5 unused bits */</span>
    <span style="color:#204a87;font-weight:bold">char</span> <span style="color:#000">buf</span><span style="color:#000;font-weight:bold">[</span><span style="color:#000;font-weight:bold">]</span><span style="color:#000;font-weight:bold">;</span>
<span style="color:#000;font-weight:bold">}</span><span style="color:#000;font-weight:bold">;</span>
<span style="color:#204a87;font-weight:bold">struct</span> <span style="color:#000">__attribute__</span> <span style="color:#000;font-weight:bold">(</span><span style="color:#000;font-weight:bold">(</span><span style="color:#000">__packed__</span><span style="color:#000;font-weight:bold">)</span><span style="color:#000;font-weight:bold">)</span> <span style="color:#000">sdshdr64</span> <span style="color:#000;font-weight:bold">{</span>
    <span style="color:#000">uint64_t</span> <span style="color:#000">len</span><span style="color:#000;font-weight:bold">;</span> <span style="color:#8f5902;font-style:italic">/* used */</span>
    <span style="color:#000">uint64_t</span> <span style="color:#000">alloc</span><span style="color:#000;font-weight:bold">;</span> <span style="color:#8f5902;font-style:italic">/* excluding the header and null terminator */</span>
    <span style="color:#204a87;font-weight:bold">unsigned</span> <span style="color:#204a87;font-weight:bold">char</span> <span style="color:#000">flags</span><span style="color:#000;font-weight:bold">;</span> <span style="color:#8f5902;font-style:italic">/* 3 lsb of type, 5 unused bits */</span>
    <span style="color:#204a87;font-weight:bold">char</span> <span style="color:#000">buf</span><span style="color:#000;font-weight:bold">[</span><span style="color:#000;font-weight:bold">]</span><span style="color:#000;font-weight:bold">;</span>
<span style="color:#000;font-weight:bold">}</span><span style="color:#000;font-weight:bold">;</span></code></pre></div>
<div  align="center"><img src="https://wejack639.oss-cn-beijing.aliyuncs.com/blogimages/img/20221104105340.png" width = 500 /></div>

<div  align="center"><img src="https://wejack639.oss-cn-beijing.aliyuncs.com/blogimages/img/20221104105425.png" width = 600 /></div>

<p>一般情况下windows64位一般使用LLP64模型，64位Unix，Linux使用的是LP64模型</p>

<table>
<thead>
<tr>
<th align="left">Datetype</th>
<th align="left">LP64</th>
<th align="left">ILP64</th>
<th align="left">LLP64</th>
<th align="left">ILP32</th>
<th align="left">LP32</th>
</tr>
</thead>

<tbody>
<tr>
<td align="left">char</td>
<td align="left">8</td>
<td align="left">8</td>
<td align="left">8</td>
<td align="left">8</td>
<td align="left">8</td>
</tr>

<tr>
<td align="left">short</td>
<td align="left">16</td>
<td align="left">16</td>
<td align="left">16</td>
<td align="left">16</td>
<td align="left">16</td>
</tr>

<tr>
<td align="left">int</td>
<td align="left">32</td>
<td align="left">64</td>
<td align="left">32</td>
<td align="left">32</td>
<td align="left">16</td>
</tr>

<tr>
<td align="left">long</td>
<td align="left">64</td>
<td align="left">64</td>
<td align="left">32</td>
<td align="left">32</td>
<td align="left">32</td>
</tr>

<tr>
<td align="left">long long</td>
<td align="left">64</td>
<td align="left">64</td>
<td align="left">64</td>
<td align="left">64</td>
<td align="left">64</td>
</tr>

<tr>
<td align="left">pointer</td>
<td align="left">64</td>
<td align="left">64</td>
<td align="left">64</td>
<td align="left">32</td>
<td align="left">32</td>
</tr>
</tbody>
</table>

<p>参考：</p>

<p><a href="https://juejin.cn/post/7074538808259117064">https://juejin.cn/post/7074538808259117064</a></p>

<p><a href="https://blog.csdn.net/wejack/article/details/127686029">https://blog.csdn.net/wejack/article/details/127686029</a></p>

<p><a href="https://blog.csdn.net/weixin_40997360/article/details/79948968">https://blog.csdn.net/weixin_40997360/article/details/79948968</a></p>

<p><a href="https://zhuanlan.zhihu.com/p/140063999">https://zhuanlan.zhihu.com/p/140063999</a></p>

<p><a href="https://juejin.cn/post/6922352059517779976">https://juejin.cn/post/6922352059517779976</a></p>

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